268 lines
10 KiB
Plaintext
268 lines
10 KiB
Plaintext
CEC Kernel Support
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==================
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The CEC framework provides a unified kernel interface for use with HDMI CEC
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hardware. It is designed to handle a multiple types of hardware (receivers,
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transmitters, USB dongles). The framework also gives the option to decide
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what to do in the kernel driver and what should be handled by userspace
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applications. In addition it integrates the remote control passthrough
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feature into the kernel's remote control framework.
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The CEC Protocol
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----------------
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The CEC protocol enables consumer electronic devices to communicate with each
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other through the HDMI connection. The protocol uses logical addresses in the
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communication. The logical address is strictly connected with the functionality
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provided by the device. The TV acting as the communication hub is always
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assigned address 0. The physical address is determined by the physical
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connection between devices.
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The CEC framework described here is up to date with the CEC 2.0 specification.
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It is documented in the HDMI 1.4 specification with the new 2.0 bits documented
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in the HDMI 2.0 specification. But for most of the features the freely available
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HDMI 1.3a specification is sufficient:
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http://www.microprocessor.org/HDMISpecification13a.pdf
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The Kernel Interface
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====================
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CEC Adapter
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-----------
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The struct cec_adapter represents the CEC adapter hardware. It is created by
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calling cec_allocate_adapter() and deleted by calling cec_delete_adapter():
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struct cec_adapter *cec_allocate_adapter(const struct cec_adap_ops *ops,
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void *priv, const char *name, u32 caps, u8 available_las,
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struct device *parent);
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void cec_delete_adapter(struct cec_adapter *adap);
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To create an adapter you need to pass the following information:
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ops: adapter operations which are called by the CEC framework and that you
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have to implement.
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priv: will be stored in adap->priv and can be used by the adapter ops.
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name: the name of the CEC adapter. Note: this name will be copied.
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caps: capabilities of the CEC adapter. These capabilities determine the
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capabilities of the hardware and which parts are to be handled
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by userspace and which parts are handled by kernelspace. The
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capabilities are returned by CEC_ADAP_G_CAPS.
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available_las: the number of simultaneous logical addresses that this
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adapter can handle. Must be 1 <= available_las <= CEC_MAX_LOG_ADDRS.
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parent: the parent device.
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To register the /dev/cecX device node and the remote control device (if
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CEC_CAP_RC is set) you call:
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int cec_register_adapter(struct cec_adapter *adap);
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To unregister the devices call:
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void cec_unregister_adapter(struct cec_adapter *adap);
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Note: if cec_register_adapter() fails, then call cec_delete_adapter() to
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clean up. But if cec_register_adapter() succeeded, then only call
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cec_unregister_adapter() to clean up, never cec_delete_adapter(). The
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unregister function will delete the adapter automatically once the last user
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of that /dev/cecX device has closed its file handle.
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Implementing the Low-Level CEC Adapter
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--------------------------------------
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The following low-level adapter operations have to be implemented in
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your driver:
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struct cec_adap_ops {
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/* Low-level callbacks */
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int (*adap_enable)(struct cec_adapter *adap, bool enable);
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int (*adap_monitor_all_enable)(struct cec_adapter *adap, bool enable);
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int (*adap_log_addr)(struct cec_adapter *adap, u8 logical_addr);
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int (*adap_transmit)(struct cec_adapter *adap, u8 attempts,
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u32 signal_free_time, struct cec_msg *msg);
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void (*adap_log_status)(struct cec_adapter *adap);
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/* High-level callbacks */
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...
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};
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The three low-level ops deal with various aspects of controlling the CEC adapter
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hardware:
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To enable/disable the hardware:
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int (*adap_enable)(struct cec_adapter *adap, bool enable);
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This callback enables or disables the CEC hardware. Enabling the CEC hardware
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means powering it up in a state where no logical addresses are claimed. This
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op assumes that the physical address (adap->phys_addr) is valid when enable is
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true and will not change while the CEC adapter remains enabled. The initial
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state of the CEC adapter after calling cec_allocate_adapter() is disabled.
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Note that adap_enable must return 0 if enable is false.
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To enable/disable the 'monitor all' mode:
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int (*adap_monitor_all_enable)(struct cec_adapter *adap, bool enable);
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If enabled, then the adapter should be put in a mode to also monitor messages
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that not for us. Not all hardware supports this and this function is only
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called if the CEC_CAP_MONITOR_ALL capability is set. This callback is optional
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(some hardware may always be in 'monitor all' mode).
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Note that adap_monitor_all_enable must return 0 if enable is false.
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To program a new logical address:
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int (*adap_log_addr)(struct cec_adapter *adap, u8 logical_addr);
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If logical_addr == CEC_LOG_ADDR_INVALID then all programmed logical addresses
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are to be erased. Otherwise the given logical address should be programmed.
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If the maximum number of available logical addresses is exceeded, then it
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should return -ENXIO. Once a logical address is programmed the CEC hardware
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can receive directed messages to that address.
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Note that adap_log_addr must return 0 if logical_addr is CEC_LOG_ADDR_INVALID.
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To transmit a new message:
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int (*adap_transmit)(struct cec_adapter *adap, u8 attempts,
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u32 signal_free_time, struct cec_msg *msg);
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This transmits a new message. The attempts argument is the suggested number of
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attempts for the transmit.
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The signal_free_time is the number of data bit periods that the adapter should
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wait when the line is free before attempting to send a message. This value
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depends on whether this transmit is a retry, a message from a new initiator or
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a new message for the same initiator. Most hardware will handle this
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automatically, but in some cases this information is needed.
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The CEC_FREE_TIME_TO_USEC macro can be used to convert signal_free_time to
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microseconds (one data bit period is 2.4 ms).
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To log the current CEC hardware status:
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void (*adap_status)(struct cec_adapter *adap, struct seq_file *file);
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This optional callback can be used to show the status of the CEC hardware.
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The status is available through debugfs: cat /sys/kernel/debug/cec/cecX/status
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Your adapter driver will also have to react to events (typically interrupt
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driven) by calling into the framework in the following situations:
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When a transmit finished (successfully or otherwise):
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void cec_transmit_done(struct cec_adapter *adap, u8 status, u8 arb_lost_cnt,
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u8 nack_cnt, u8 low_drive_cnt, u8 error_cnt);
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The status can be one of:
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CEC_TX_STATUS_OK: the transmit was successful.
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CEC_TX_STATUS_ARB_LOST: arbitration was lost: another CEC initiator
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took control of the CEC line and you lost the arbitration.
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CEC_TX_STATUS_NACK: the message was nacked (for a directed message) or
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acked (for a broadcast message). A retransmission is needed.
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CEC_TX_STATUS_LOW_DRIVE: low drive was detected on the CEC bus. This
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indicates that a follower detected an error on the bus and requested a
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retransmission.
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CEC_TX_STATUS_ERROR: some unspecified error occurred: this can be one of
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the previous two if the hardware cannot differentiate or something else
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entirely.
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CEC_TX_STATUS_MAX_RETRIES: could not transmit the message after
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trying multiple times. Should only be set by the driver if it has hardware
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support for retrying messages. If set, then the framework assumes that it
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doesn't have to make another attempt to transmit the message since the
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hardware did that already.
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The *_cnt arguments are the number of error conditions that were seen.
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This may be 0 if no information is available. Drivers that do not support
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hardware retry can just set the counter corresponding to the transmit error
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to 1, if the hardware does support retry then either set these counters to
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0 if the hardware provides no feedback of which errors occurred and how many
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times, or fill in the correct values as reported by the hardware.
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When a CEC message was received:
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void cec_received_msg(struct cec_adapter *adap, struct cec_msg *msg);
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Speaks for itself.
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Implementing the High-Level CEC Adapter
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---------------------------------------
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The low-level operations drive the hardware, the high-level operations are
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CEC protocol driven. The following high-level callbacks are available:
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struct cec_adap_ops {
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/* Low-level callbacks */
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...
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/* High-level CEC message callback */
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int (*received)(struct cec_adapter *adap, struct cec_msg *msg);
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};
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The received() callback allows the driver to optionally handle a newly
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received CEC message
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int (*received)(struct cec_adapter *adap, struct cec_msg *msg);
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If the driver wants to process a CEC message, then it can implement this
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callback. If it doesn't want to handle this message, then it should return
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-ENOMSG, otherwise the CEC framework assumes it processed this message and
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it will not no anything with it.
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CEC framework functions
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-----------------------
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CEC Adapter drivers can call the following CEC framework functions:
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int cec_transmit_msg(struct cec_adapter *adap, struct cec_msg *msg,
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bool block);
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Transmit a CEC message. If block is true, then wait until the message has been
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transmitted, otherwise just queue it and return.
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void cec_s_phys_addr(struct cec_adapter *adap, u16 phys_addr, bool block);
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Change the physical address. This function will set adap->phys_addr and
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send an event if it has changed. If cec_s_log_addrs() has been called and
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the physical address has become valid, then the CEC framework will start
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claiming the logical addresses. If block is true, then this function won't
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return until this process has finished.
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When the physical address is set to a valid value the CEC adapter will
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be enabled (see the adap_enable op). When it is set to CEC_PHYS_ADDR_INVALID,
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then the CEC adapter will be disabled. If you change a valid physical address
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to another valid physical address, then this function will first set the
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address to CEC_PHYS_ADDR_INVALID before enabling the new physical address.
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int cec_s_log_addrs(struct cec_adapter *adap,
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struct cec_log_addrs *log_addrs, bool block);
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Claim the CEC logical addresses. Should never be called if CEC_CAP_LOG_ADDRS
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is set. If block is true, then wait until the logical addresses have been
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claimed, otherwise just queue it and return. To unconfigure all logical
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addresses call this function with log_addrs set to NULL or with
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log_addrs->num_log_addrs set to 0. The block argument is ignored when
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unconfiguring. This function will just return if the physical address is
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invalid. Once the physical address becomes valid, then the framework will
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attempt to claim these logical addresses.
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